It is well known to employ silencers, such as quarter-wave tuners and other resonators, to reduce air induction noise created by the air induction system of an internal combustion engine. Such air induction systems generally comprise an air cleaner and prior approaches have often employed a resonator or the like incorporated into the air cleaner or communicating with an air intake passage downstream thereof. Typically, such noise reduction systems attenuate noise at only a fixed narrow frequency range. The dominant noise frequency produced by an air induction system (or, in any event, the frequency most desirably attenuated) is different, however, at different engine operating levels. Such prior, fixed narrow frequency range noise reduction devices tuned for a first engine operating level may be substantially ineffective, or even counter-effective, at other significant frequencies, most notably at a dominant noise frequency produced by the air induction system at a second engine operating level.
Attempts to overcome the problem of fixed narrow frequency range noise reduction devices have included proposals for use of electronically actuated controls, for example, electronically actuated valves to turn a resonator on and off. A noise reduction device is shown in U.S. Pat. No. 4,538,556 to Takeda, wherein a tank communicates at two spaced ports with an air intake tube of an air induction system. One port is opened and closed by a vacuum actuated valve. The vacuum is supplied to the valve from a vacuum tank through a solenoid valve. An electronic signal, based on engine speed or other engine operating condition, actuates the solenoid valve to open or close fluid communication between the vacuum tank and the vacuum actuated valve. Electronic controls add expense and complexity to noise reduction devices for air induction systems.
A noise reduction device for the air induction system of an internal combustion engine is disclosed in Ojala et al There, a noise reduction side-branch reactive silencer is formed within a sub-frame interposed between the engine and a vehicle body. The sub-frame forms the reactive cavity and has a connector for communicating the reactive cavity with the air induction system. The connector is positioned along the sub-frame in accordance with the desired attenuation frequency. This system, while innovative and effective, is not directed to providing variable frequency noise reduction.